question

screwless avatar image
screwless asked

Which MPPT controller for panels in series?

Hi,

I have read too much and I am now utterly confused!

I have three Victron panels (each with the following specs):

215W

24V

max. V 37.4

VOC 45.82

max pw 5.75

I was going to install in parallel and use my mppt 100/50. I now think in series (645W) would be better suited for my set-up (panels on roof, no shade, UK). The victron mppt calculator suggests the mppt 250/60. I understand that the 100/50 cannot cope with the combined VOC of ~140 (3x 45.82) but 250 seems overkill? Why not the 150/60? Am I right that the amp (5.75) will remain the same?

I will have to run cable for approx. 9 meters (30 feet) and I have two 130ah AGM batteries.

Questions -

Which mppt? Do I really need the 250/60?

Cable strength if in series? The amp is pretty low - would a 8AWG be enough?

Batteries in series or parallel (12V or 24V)? What's best?

I would really appreciate some help/corrections/advice!

Thank you!!!


MPPT ControllersMPPT SmartSolarSolar Panel
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4 Answers
kevgermany avatar image
kevgermany answered ·

The calculator will factor in the rise in panel voltage at low temperatures.

Could be that you input the temperature compensation incorrectly.

But Voc in series is on or close to the limit for a 150V MPPT. And it's Voc that's used for these sizing calcs.

The other killer is usually short circuit current (Isc), but with these series panels, shouldn't be a problem.

Would be good if you redo the calc and link to it here. We can probably spot the problem.

What voltage is your battery bank?

The 24V is an indication that the panels are ok for a 24V battery bank.

Current will depend on panel power and battery voltage while charging. So a 12V battery would get charged at about 50A on either controller.


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screwless avatar image screwless commented ·

Thank you very much! I really appreciate the help.

My calculation below:

1712126083534.png

link:

https://mppt.victronenergy.com#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1712126083534.png (141.1 KiB)
kevgermany avatar image kevgermany ♦♦ screwless commented ·

Guess you saw that the max voltage at your lowest temp is over 150.

If you switch the panels to parallel, you can get away with a 100/20, so your existing MPPT is more than enough.

https://mppt.victronenergy.com/#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klim8skeptic avatar image klim8skeptic ♦ kevgermany ♦♦ commented ·
@kevgermany change that ^ to 3 strings.
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pwfarnell avatar image
pwfarnell answered ·

What temperature is your quoted Voc at. In bright sunshine on s very cold day the Voc can be higher than the figure on the panel spec which is typically quoted at 25degC. This can easily result in the Voc going above spec. On the input voltage, if it goes over it can permanently damage the MPPT so do not get too close. This is not like the current limit where the controller can regulate.

You are correct for series panels, the current will be 5.75A.

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screwless avatar image screwless commented ·
Thank you! I'm not sure about the temperature, I just took the Voc from the Victron spec for these panels. Using the 150/60 would be cutting it close I suppose, that makes more sense now. Do you think an 8AWG (10mm2) would be sufficient for the amp?
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Craig Chamberlain avatar image Craig Chamberlain screwless commented ·

10mm cable is way bigger than you need for 5.75A. Even 4mm is more than adequate but is commonly available so I’d tend to go with that. See cable specs here:

https://www.elandcables.com/media/uluhjuev/photovoltaic-solar-h1z2z2-k-cable.pdf

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klim8skeptic avatar image
klim8skeptic answered ·

3 panels in parallel will work on a 100-50.

Dont forget some fuses for each panel.

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screwless avatar image screwless commented ·

Thank you. I want to install the panels in series instead of in parallel; I believe it is better suited.

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Fideri avatar image Fideri commented ·

@klim8skeptic

Why when that exceeds the total VOC? I thought this was a hard limit?

F.

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klim8skeptic avatar image klim8skeptic ♦ Fideri commented ·

@Fideri Why when that exceeds the total VOC?

3 panels paralleled gives a Voc of 45.82v.

3 panels paralleled gives a Vmp of 37.4v. More than enough volts to properly charge any 24v battery. Even at high panel temps.

The downside of 3 paralleled panels is that fuses need to be on each lead of each panel.

Still works out cheaper than a 250-60.

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kevgermany avatar image
kevgermany answered ·

Let's get onto series/parallel for the batteries.

Basics only.

The decision is a long term commitment, unless you are ok with expensive upgrades later.

24V battery system halves currents compared to 12V battery system. Much better suited to heavier loads like inverters. Less lower loss. Lighter cables.

All loads need to be 24V. Any 12V loads need a DC:DC converter. E.g. an Orion 24/12-xx where xx is the max current.

Unless you have a lot of existing 12V loads, there's no real reason to install a 12V system.


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